The present invention relates to a method of calculating an amount of offset for a wire electrode in a wire cut electric discharge machining operation.
In the electric discharge machining operation, the amount of offset is determined according to machining conditions such as the material and diameter of the wire electrode, the material and thickness of a workpiece to be machined.
FIG. 1 is an explanatory diagram showing the entire arrangement of a conventional wire electric discharge machining apparatus.
In FIG. 1, reference numeral 50 designates a numerical control unit for controlling the wire cut electric discharge machine; 51, a machining power source unit for applying a high frequency pulse voltage across a wire electrode 52 and a workpiece 53 to be machined which is of electrically conductive material; 54 and 55, movable tables which are movable in the direction of X-axis and in the direction of Y-axis, respectively, with the workpiece fixedly positioned thereon; 56 and 57, drive motors for driving the movable tables 54 and 55 in response to instructions from the numerical control unit 50; 58 and 9, die feeders for supplying the output high frequency pulse voltage of the machining power source unit 51 to the wire electrode 52; 60, a wire winding bobbin; 61, a wire supply bobbin; and 65, a machining solution supplying device for supplying a machining solution (generally water) to the gap between the wire electrode 52 and the workpiece 53.
FIG. 2 is a schematic diagram illustrating a sectional view of the wire electrode 52 and the workpiece 53 in the case where a die is formed by machining the workpiece 53 four times (four machining steps). In FIG. 2, reference character H.sub.1 designates the amount of an offset in the first machining steps, and 74, the locus of the center axis of the wire electrode 52. Similarly, in FIG. 2, reference characters H.sub.2, H.sub.3 and H.sub.4 designate the amounts of offset in the second, third and fourth machining steps, respectively; and 75, 76 and 77, the loci of the center axis of the wire electrode 52 in the machining steps, respectively.
FIG. 3 is a table indicating the amounts of offset in the first, second, third and fourth machining steps.
The operation of the wire cut electric discharge machining apparatus thus organized will be described hereinafter.
In response to an instruction provided according to the machining pattern program stored in the numerical control unit 50, the movable tables 54 and 55 are moved in the direction of X-axis and in the direction of Y-axis so that the wire electrode 52 and the workpiece 53 are moved relative to each other so that the workpiece is machined with the discharge energy to obtain an aimed two-dimensional contour, while a discharge machining pulse voltage is applied across the wire electrode 52 and the workpiece 53. In this operation, the machining solution supplying device 65 applies the machining solution to the discharge gap between the wire electrode 52 and the workpiece 53, in order to electrically insulate the wire electrode 52 and the workpiece 53 from each other, to cool down the wire electrode 52 and the workpiece 53, and to remove sludge formed by electric discharge.
The aimed two-dimensional contour is obtained by electric discharge machining the workpiece as described above. In this case, the machining operation should be carried out along the locus which is shifted from the NC program locus by the sum of the wire radius and the discharge gap. This is the aforementioned amount of offset. The amount of offset depends on machining conditions and changes every machining operation. Therefore, every machining operation it is necessary to read the amount of offset (hereinafter referred to as "offset data", when applicable) from a machining condition table or the like. This will be described with reference to FIGS. 2 and 3.
As was described above, FIG. 2 shows a sectional view of the wire electrode 52 and the workpiece 53 in the case where a die is formed by machining the workpiece four times. In the first machining step, the amount of offset is set to H.sub.1, so that the center axis of the wire electrode 52 goes along the locus 74 to provide a machined surface 78. Then, a finishing machining operation including second, third and fourth machining steps is effected. That is, in the second machining step, the amount of offset is set to H.sub.2, so that the center axis of the wire electrode 52 moves along the locus 75, to form a machined surface 79. Similarly, in the third and fourth machining steps, the amount of offset is set to H.sub.3 and H.sub.4. Thus, finally, a finished surface 81 is obtained.
In the conventional method of setting the amount of offset, when the number of machining steps (four in the above description) changes, the amount of offset for each machining step is changed. That is, in the case where, in FIG. 2, a machining operation is carried out only once to complete the final aimed die under the same machining condition as the first machining step, the machined surface 78 can be regarded as the final finished surface 81. In this case, the distance between the surface 78 and the center axis of the wire electrode 52 is employed as the amount of offset, then the aimed pattern can be formed by machining the workpiece only once. However, it should be noted that, in this operation, the amount of offset is the sum of the radius of the wire electrode 52 and the discharge gap, and therefore it cannot be obtained from the amounts of offset H.sub.1 through H.sub.4 in the four machining steps described above, because the discharge gap width is changeable depending on the discharge conditions. Similarly, in the case where a finished surface is obtained by machining the workpiece twice, the machined surface 79 can be regarded as the finished surface, and the amount of offset in the first machining step is the sum of the finishing margin f.sub.2 in the second machining step, the wrre radius, and the discharge gap in the first machining step. In the second machining operation, the amount of offset is the sum of the wire radius and the discharge gap in the second machining step, and therefore it cannot be obtained directly from the data H.sub.1 through H.sub.4, because the finishing margins f.sub.2 through f.sub.4 and the discharge gaps in the machining operations cannot be determined.
As is apparent from the above description, in the case where the workpiece is finished by only one machining step or plural machining steps, the amounts of offset for all the machining steps must be stored in combination, although the same machining conditions are employed for the first through fourth machining steps. Furthermore, in machining a workpiece high in residual strain, sometimes a machining operation is carried out several times under the same machining conditions. In such a case, the conventional method of setting an amount of offset is not practical. If it is forcibly employed, then data necessary for determining an amount of offset will be considerably large in quantity.